Optimizing Floating Offshore Multi-Wind-Turbine Design: A Parametric Study on Tower Inclination and Column Spacing

摘要

Multi-turbine floating offshore platforms (MUFOPs) are emerging as a viable concept for reducing levelized cost of energy in offshore wind developments. If properly designed, the cost per megawatt of electric power generated can be lower compared to single-turbine platforms. To maximize yield, minimize cost and ensure a safe design, the spacing between rotor-nacelle assemblies (RNAs), must be carefully considered. This spacing (R), is the sum of the platform column spacing (c), and tower horizontal projected length (h). Rotor diameters pose considerable challenges to the arrangement of multiple wind turbines on one platform; challenges pertaining to safe operations, feasibility of construction and transportation. Specific insights are necessary to facilitate the development of viable concepts. The parametric study presented in this paper discusses the optimization of MUFOPs using tower inclination and column spacing. Representative configurations (adjusting tower inclination and / or column spacing) are developed with a multi-turbine semi-submersible-type platform and analyzed in time domain using coupled analysis. The configurations consist of two 5 WM reference turbines of the National Renewable Energy Laboratory (NREL), U.S.A. A non-dimensional parameter (R/c), is used to characterize the configurations. Wind, wave, and current loads are applied in analysis to assess the behavior of the system holistically. Hydrodynamic, aerodynamic, elasto-dynamic, servo-dynamic and mooring-dynamic effects are captured interactively at each time-step of analysis. An operational turbine condition is simulated in analysis using full-field turbulent wind to capture spatial variations of wind loads acting on each turbine of the system. Characteristic responses of the nacelle, tower and platform are assessed to determine the optimal combination which avoids both inadequate and excessively conservative designs of multi-wind-turbine platforms. By analyzing the spectral densities of the responses, the potential impact of the observed responses on fatigue design is qualified. Optimal configurations from the scenarios considered, allow minimal or no wake interactions, tolerable towerbase loads and acceptable accelerations and motions of the nacelle and platform. Results indicate that optimal solutions exist at R/c ratios greater than 1.0. An assessment of a range of tower inclinations and column spacing for optimal design of multi-wind-turbine platforms is a study that has not been documented in literature, and deserves attention considering current industry trends for floating offshore wind turbines. This paper offers significant insights on the characteristics of optimal tower and column arrangements for such platforms and provides reliable benchmarks for future designs.